Photoinitiated polymerization has attracted a great deal of attention for potential applications in biomaterials, surface coatings, printing inks, stereolithography, and nanocomposites. Different from conventional polymerization using thermal initiation, photoinitiating systems can be performed under milder conditions and at lower temperatures. These features offer a feasible approach for stereospecific polymerization, biomaterial synthesis, and efficient production, as side reactions are reduced and energy production is more cost efficient. The vast majority of current commercial applications use free radical photoinitiators (e.g., benzoins, acetophenones, and aromatic ketones) and cationic photoinitiators (salts such as diaryliodonium salts, N-ethoxy-2-methylpyridinium and triarylsulphonium salts).
Although free radical and cationic systems have been widely investigated and applied in industry, few reports of anionic photoinitiating species exist. Kutal et al. reported photopolymerization of ethyl cyanoacrylate (ECA) using thiocyanate ions (NCS−) as anionic species generated from ligand exchange of metal Cr-complex (Scheme 1).

Subsequently, several examples of anionic photoinitiators were also developed, such as the acac anion formed from Pt(acac)2, pyridine from Cr(CO)5Py, triphenylphosphine from heterolytic cleavage of phosphonium salts, and metallocenes. However, most of the reported anionic photoinitiating systems consist of organometallic complexes or generation of Lewis bases from the corresponding salts.
The characteristics of controlled, propagating anionic polymerization lacking significant chain transfer or a chain termination step can provide a robust and versatile synthetic route for production of well-defined, low-polydispersity polymers. However, few successes have been reported using photochemical methods, even despite the fact that photopolymerization has gained attention over the past few decades and conventional anionic polymerization has been developing since early 1910. For example, Manners et al. demonstrated a living photopolymerization by irradiating metal-containing ferrocenophane monomers using an anionic initiator.